The invention relates to microwave phase shifting structures, and more particularly to wave transmission structures providing differential phase shift between two waves polarized orthoganally to each other.
Structures providing differential phase shift between two orthogonal linear polarizations have a variety of applications. The most common application is for circular polarizers in which the differential phase shift is 90.degree. (quarter-wave plate). A differential phase shift of 180.degree. (half-wave plate) is used as a polarization rotator for linear polarization and as a phase shifter for circular polarization, e.g., Fox, A. G., "An Adjustable Waveguide Phase Changer," PROC. IRE, Vol. 35, No. 12, pp. 1489-1498, December 1947. In conjunction with orthopolarization mode transducers they can be used as power dividers. These structures may also be used for a single polarization as fixed phase shifters.
Conventional differential phase shift structures are understood to employ periodic lumped or distributed shunt capacitive or periodic lumped or distributed shunt inductive loading in the differential phase shift region which is inherently mismatched with the unloaded waveguide; hence an impedance matching section is required at each end of the phase shift section. One conventional design is illustrated in the paper "Phase Shift by Periodic Loading of Waveguide and Its Application to Broad-Band Circular Polarization," by A. J. Simmons, IRE Transactions, Microwave Theory and Techniques, December, 1955, pages 18-21. Other designs are illustrated in "Microwave Transmission Circuits," edited by George L. Ragan, MIT Rad. Lab Series Volume 9. FIGS. 6.59-6.63 illustrate various configurations employing shunt capacitive fin loading for a quarter-wave plate circular polarizer, shunt inductive loading in a quarter-wave plate circular polarizer, and an array of shunt capacitive posts in a differential phase shift section. FIG. 6.69 illustrates two designs employing capacitive dielectric slabs.
However, none of these prior methods use shunt capacitive and series inductive loading in the same structure and in the proper ratio to achieve impedance matching to the unloaded waveguide and at the same time achieve greater differential phase shift per unit length, thus obviating the need for impedance transformers at each phase shift section.
It would therefore be advantageous to provide a structure for achieving a differential phase shift between two waves polarized orthogonally to each other, and which is impedance matched between the unloaded waveguide and the phase shifting section for both components of polarization. Such a structure would not require impedance transformer sections at each end of the phase shift section, thereby reducing the overall length and complexity of the structure.
It would further represent an advance in the art to provide an easily fabricated, differential phase shift per unit length structure which provides a relatively large differential phase shift per unit length, with low insertion loss over a relatively large bandwidth.